1. Field of the Invention
The present invention relates to a heat treatment method and to a heat treatment apparatus applying the heat treatment method. In particular, the present invention relates to a method of heat treatment, and a heat treatment apparatus, in which a gas is heated by thermal radiation from a heat generator such as a lamp, and then a substrate to be processed, or formations on the substrate to be processed, are then heated by using the heated gas.
2. Description of the Related Art
Heat treatment processes for objectives such as oxidation, diffusion, gettering, and crystallization after ion injection with respect to semiconductors and semiconductor substrates are incorporated into methods of manufacturing semiconductor devices. Horizontal and vertical hot wall type annealing furnaces are typical examples of apparatuses for performing such heat treatment.
Horizontal and vertical annealing furnaces are batch type devices for processing a plurality of substrates at a time. For example, substrates are loaded horizontally and in parallel into a susceptor made from quartz with vertical annealing furnaces, and the substrates are taken into, and removed from, a reaction chamber by using a vertical motion elevator. Heaters are disposed in the periphery of a bell jar type reaction chamber, and the substrates are heated by using the heaters. The amount of time necessary for heat up until achieving a predetermined heat treatment temperature, and the amount of time necessary for cooling to a temperature at which it is possible to remove the substrates, are relatively long.
However, extremely precise process accuracy are required for devices such as MOS transistors used in integrated circuits due to their miniaturization. In particular, it is necessary to keep the diffusion of impurities to the minimum possible when forming shallow junctions. It is difficult to form shallow junctions with furnace annealing processes that require a lot of time for heat up and cool down.
Rapid thermal annealing (hereafter referred to as RTA) has been developed as a heat treatment technique for performing rapid heating and rapid cooling. With RTA, substrates or formations on the substrates are heated quickly using means such as an infrared lamp, making it possible to perform heat treatment in a short amount of time.
On the other hand, the use of thin film transistors (hereafter referred to as TFTs) has been gathering attention as a technique in which it is possible to form integrated circuits directly on a glass substrate. Development of applications of this technique to new electronic devices such as liquid crystal display devices has been advancing. In particular, heat treatment for activation and in order to relieve distortions is necessary for TFTs in which impurity regions such as source and drain regions are formed in a polycrystalline semiconductor film formed over a glass substrate. However, the distortion point of glass substrates is at best from 600 to 700° C., they have poor heat resistance, and easily break due to thermal shocks, which is a disadvantage.
With a conventional vertical or horizontal annealing furnace, it becomes difficult to ensure a uniform heat treatment temperature, if the substrate size is made large size, whether the substrates for forming integrated circuits are semiconductors or materials like glass or ceramics. In order to ensure a uniform temperature within the substrates, and between the substrates, it is necessary to consider the fluid characteristics of a gas flowing in the reaction chamber and to widen the gap (pitch) between the substrates to be processed, which are arranged horizontally and in parallel. For example, if one side of a substrate exceeds 500 mm in length, it then becomes necessary to open a substrate gap equal to or greater than 30 mm.
The apparatus size therefore necessarily becomes very large if the substrates to be processed are made large size. Further, a large amount of substrates are processed in one batch, thus the weight increases, and it is necessary to use a strengthened susceptor in order to hold the substrates to be processed. The weight thus further increases, and mechanical operation for conveying the substrates to be processed into and out of the apparatus becomes slower. Also, not only does the floor surface area of the heat treatment apparatus increase, it also even influences architectural costs for a building in order to ensure that its floor can withstand the load. The increased apparatus size thus starts a vicious cycle.
On the other hand, there is a prerequisite for sheet processing with the RTA method, and there is no extreme increase in the weight of the apparatus. However, differences in the absorption of lamp light, used as a heat treatment means, develop due to the characteristics of the substrates to be processed, and the formations on the substrates. For example, for a case in which a metallic wiring pattern is formed on a glass substrate, a phenomenon arises in which the metallic wirings are heated preferentially, localized distortions develop, and the glass substrates break. It is therefore necessary to have complex control in the heat treatment process, such as with the regulation of the speed of temperature increase.